Summary of Significant Accounting Policies (Recent accounting change) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other operating costs and expenses									$ 2,436,932	$ 2,395,619	$ 2,289,750
Income before income taxes									772,770	896,438	732,030
Federal and foreign income taxes									(219,433)	(292,953)	(227,923)
Net income	$ 154,589	$ 162,054	$ 109,004	$ 123,447	$ 152,790	$ 220,650	$ 108,967	$ 119,511	$ 549,094	$ 601,916	$ 503,694
Basic net income per share (dollars per share)	$ 1.22	$ 1.29	$ 0.87	$ 1.01	$ 1.26	$ 1.80	$ 0.89	$ 0.97	$ 4.40	$ 4.91	$ 4.06
Diluted net income per share (dollars per share)	$ 1.21	$ 1.26	$ 0.85	$ 0.96	$ 1.20	$ 1.72	$ 0.85	$ 0.93	$ 4.26	$ 4.68	$ 3.87